1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member, an electrophotographic apparatus, and an electrophotographic method and, more particularly, to an electrophotographic photosensitive member, which is a light receiving member, an electrophotographic apparatus, and an electrophotographic method capable of providing high-quality images stably throughout a long period of time without image unfocussing or image smearing.
2. Related Background Art
Hitherto, there have been known many electrophotographic methods, for example, as described in U.S. Pat. No. 2,297,692, Japanese Patent Publication No. 42-23910, and Japanese Patent Publication No. 43-24748. It is common practice to utilize a light receiving member, form an electric latent image on the light receiving member by various means, then develop the latent image with a developing agent (developer), electrically transfer the developer image onto a transfer medium such as paper as occasion demands, and thereafter fix the image by heat, pressure, heat and pressure, or solvent vapor or the like to obtain a copy.
In the above steps, since the residual developer remains on the surface of the light receiving member even after the developer image has been transferred onto the transfer medium, a cleaning blade, used as a means for removing the residual developer, is put in contact with the surface of the light receiving member to scrape the residual developer therefrom and discharge the untransferred developer to the outside of the system.
As the materials for the light receiving member used as an electrophotographic photosensitive member, a variety of materials are suggested, including inorganic materials such as selenium, cadmium sulfide, zinc oxide, and amorphous silicon (hereinafter referred to as a-Si), organic materials, and so on. Of these materials, non-monocrystalline deposited films containing silicon atoms as a main component, typified by a-Si, for example amorphous deposited films of a-Si or the like containing hydrogen and/or halogen (for example, fluorine, chlorine, etc.) (for example, compensating for hydrogen or dangling bonds), are suggested as high-performance, high-durability, and nonpolluting photosensitive members and some of them are practically used. U.S. Pat. No. 4,265,991 discloses the technology of the electrophotographic photosensitive member, the photoconductive layer of which is formed mainly of a-Si. Japanese Patent Application Laid-Open No. 60-12554 discloses a surface layer containing carbon and halogen atoms in the surface of a photoconductive layer comprised of amorphous silicon containing silicon atoms, and Japanese Patent Application Laid-Open No. 2-111962 discloses a photosensitive member having a surface protecting-lubricating layer provided on an a-Si:H or a-C:H photosensitive layer. These all are techniques for enhancing water repellency and wear resistance and include no description concerning the relationship between the electrophotographic process and the scraping property of the surface layer.
Since the a-Si base photosensitive members, typified by a-Si, have excellent properties which demonstrate their high sensitivity to light of long wavelengths such as semiconductor lasers (770 nm to 800 nm) and have little deterioration recognized after repetitive use, they are widely used as photosensitive members for electrophotography, for example, in high-speed copying machines, LBPs (laser beam printers), and so on.
As the methods for forming the silicon base non-monocrystalline deposited films, there are many known methods, including the sputtering method, the method of decomposing a source gas by heat (thermal CVD method), the method of decomposing a source gas by light (photo CVD method), the method of decomposing a source gas by plasma (plasma CVD method), and so on. Of these methods, the plasma CVD method, which is a method of decomposing a source gas by a glow discharge or the like generated by direct current, high frequency (RF or VHF), or microwave to form a deposited film on a desired substrate such as glass, quartz, a heat-resistant synthetic resin film, stainless steel, or aluminum are now proceeding to practical use, including not only the method of forming the amorphous silicon deposited films for electrophotography, but also methods for forming deposited films for the other uses. There are also proposed various apparatuses for such methods.
Further, in the field of the application to the electrophotographic photosensitive members, demands for improvement in quality of film and processing performance are becoming stronger and stronger in recent years and a variety of ideas are also under study.
Particularly, the plasma processes using high-frequency power are used because of their various advantages including high stability of discharge, the capability of being also used for formation of insulating materials such as oxide films or nitride films, and so on.
For the light receiving members, there are recently required improvement in the electrophotographic characteristics matching with high-speed operation and more vivid image quality. Therefore, in addition to the improvement in the characteristics of the photosensitive member, the grain diameters of the developer are being decreased and there are frequently used those developers having a weight average grain diameter of 5 to 8 .mu.m measured by a Coulter counter or the like.
Since the a-Si base light receiving members have surface hardnesses much higher than those of the other photosensitive members, a blade-type cleaning method with high cleaning ability is popularly used as a cleaning means.
However, in such a blade-type cleaning method, differences occur in the amounts of the developer remaining on the blade surface because of differences in character patterns in an original chart. Further, uneven scraping may occur in the surface layer of the light receiving member. When such uneven scraping occurs, sensitivity irregularities appear as electrophotographic characteristics and result in density irregularities in an image. This phenomenon becomes more prominent particularly as the grain diameters of the developer decrease. In recent years, because the decrease of the grain diameters of the developer is being advanced in order to meet the demands for higher quality of image characteristics, such density irregularities occur more readily.
Further, the decrease of grain diameters of the developer improves the quality of image on one hand while tending to increase scrubbing force on the other hand. This increase of scrubbing force causes the developer (toner) to slip through the cleaning blade because of chatter or the like of the cleaning blade and this slipping of the developer may cause a black-line-like cleaning failure. When the copying step is repeated in this state, fine particles of the developer and additives (strontium titanate, silica, etc.) contained in the developer may be scattered in a corona charger to adhere to a wire electrode of the corona charger (hereinafter referred to as a charger wire), thereby causing discharge irregularities. When the discharge irregularities due to the contamination of the charger wire are caused, in the case of positive development (a method of developing unexposed portions of the surface of the light receiving member), the quality of output image may be lowered by appearance of linear blank area portions on the image, scale-like black fog spreading over the entire image, local black dots (0.1 to 0.3 mm.phi.) without periodicity, and so on.
Further, when the contamination of the charger wire is caused, abnormal discharge may be induced between the contaminated portion and the light receiving member, thus damaging the surface of the photosensitive member and causing image defects.
In addition, when the frictional resistance is high, friction heat is built up between the light receiving member and the cleaning blade, and this friction heat may cause a fusion phenomenon in which the developer used for thermal fixation firmly adheres to the surface of the light receiving member. Particularly, this fusion phenomenon becomes more prominent in proportion to the decrease of grain diameters of the developer. In the first stage the fusion phenomenon is too weak to affect the image; but repetitive use makes seeds of small areas of fused developer, gradually grows them and at last causes black-line-like image defects.
As the methods for solving the problems as described above, there are included a method of increasing the urging pressure of the cleaning blade, a method of increasing the hardness of the elastic rubber blade, and so on. However, these methods increase the friction force between the blade and the surface of the light receiving member, which may promote the uneven scraping of the surface layer. Further, the method of increasing the hardness of the blade may pose a problem that the material of the blade becomes fragile, whereby the lifetime of the blade is shortened.
As a countermeasure against such uneven scraping, there has hitherto been sometimes employed a method employing a means for providing a magnetic roller or a cleaning roller of urethane rubber, silicone rubber, or the like to uniformly spread the developer to reach the cleaning blade, thereby relaxing retention irregularities of the developer on the blade surface.
Another important role of the above magnetic roller or cleaning roller of urethane rubber, silicone rubber, or the like is to remove corona discharge products on the surface of the light receiving member.
The corona discharge products include nitrogen oxides (NOx) formed by oxidation of nitrogen in the air with ozone generated in corona discharge. Further, these nitrogen oxides react with water in the air to form nitric acid and other products. The products due to the corona discharge such as the nitrogen oxides, nitric acid, etc., adhere to and are deposited on the surface of the light receiving member and peripheral devices to contaminate their surfaces.
The corona discharge products have a strong hygroscopic property and the surface of the light receiving member adsorbing them substantially decreases its charge retaining capability throughout or in part of the surface because of the decrease of the resistance of the surface of the light receiving member caused by moisture absorption of the corona discharge products deposited thereon, which will be the cause of the image defect called image smearing (the charge in the surface of the light receiving member leaks in the plane directions to destroy or impede formation of an electrostatic latent image pattern).
Further, the corona discharge products adhering to the internal surface of a shield plate of the corona charger are evaporated and liberated not only during operation of the electrophotographic apparatus but also during quiescent periods of the apparatus, e.g. during the nighttime, and they then adhere to the surface of the light receiving member at a part thereof corresponding to the discharge aperture region of the charger. Since these corona discharge products absorb moisture to decrease the resistance of the surface of the light receiving member, it becomes easier to cause the image smearing called charger trace smearing in the first one or several copies outputted when restarting the operation after a long quiescent period of the electrophotographic apparatus, at the part of the light receiving member surface corresponding to the aperture region of the charger during the above quiescent period of the apparatus.
As a countermeasure for preventing this image smearing phenomenon, there has been provided a means for heating the surface of the light receiving member at about 30 to 50.degree. C. by a heater for heating the light receiving member, a means for sending air to the light receiving member by a hot air sending device, or the like, in combination with the scrubbing means such as the cleaning roller, etc. described above. This heating means is sometimes used to lower the relative humidity to evaporate the corona discharge products adhering to the surface of the light receiving member and the water absorbed by the corona discharge products, thereby preventing the substantial decrease of the resistance of the surface of the light receiving member.
However, this heating means may cause image density irregularities of dark portions and light portions partially in image density at the period of rotation of a rotationally cylindrical developer carrier, where the size of the light receiving member and the thickness of the conductive substrate of the light receiving member are decreased with decrease in the size and cost of electrophotographic apparatus. The reason is that during the quiescent period of apparatus the heat of the light receiving member expands the rotationally cylindrical developer carrier to make irregular the distances to the facing portion of the light receiving member. The developer becomes easier to transfer in distance-shortened portions than usual.
In recent years, the tendency to personal use of copying machines and printers requires the important subjects of decrease of size, reduction of cost, and maintenance-free performance of the electrophotographic apparatuses. However, the provision of such a heating means is contrary to the requirement for the decrease of size, the reduction of cost, and the maintenance-free performance of the electrophotographic apparatuses. Further, in terms of further energy saving and ecology, the apparatus is also desirably designed without provision of the means for directly heating the light receiving member.
Moreover, in addition to the problem of image smearing, the technology for stably supplying high image quality is earnestly desired from recently growing needs for copy images. The uses of copying machines have been transferred from copy originals mainly including characters to images such as photographs, and the needs of market are increasing for copy images frequently using halftones. Therefore, severer standards than before are being demanded as to the stability of density.
Under such circumstances, there is a need for a light receiving member that does not cause image smearing and without provision of a heating means and a need for an electrophotographic apparatus that does not cause uneven scraping and that can stably supply high image quality without density irregularities under any electrophotographic process conditions.